1. Field of the Invention
This invention is related to a hysteresis comparing device, and more particularly, related to a hysteresis comparing device with constant hysteresis width.
2. Description of Related Art
Referring to FIG. 1, it is a block diagram, schematically illustrating a conventional tracking servo control loop. The tracking servo control loop is used in a CD/DVD system for reading information data stored in the CD/DVD disc and outputting the digital signals. The tracking servo control loop 100 includes a pick-up head (PUH) 102, a tracking-error signal generator 104, a low pass filter 106, and a hysteresis comparing device 108. The pick-up head 102 is used to access information signals from the CD/DVD disc, and output two satellite signals Ve and Vf. These two signals have a phase different of 180°. The tracking error signal generator 104 is coupled to the pick-up head 102 for receiving the satellite signals of Ve and Vf and then generate a tracking signal Va. The low pass filter 106 is coupled to the tracking error signal generator 104 to receive the tracking signal Va, and then generate a low pass signal Vdc in a direct current type. The low pass signal Vdc is an averaged signal level of the tracking signal Va. The hysteresis comparing device 108 has a positive input terminal, which is coupled to the tracking error signal generator 104 to receive the tracking signal Va. A negative input terminal of the hysteresis comparing device 108 is coupled to the low pass filter 106 to receive the low pass signal Vdc. The hysteresis comparing device 108 is used to receive the tracking signal Va and the low pass signal Vdc, and output the digital signal Vd.
Referring to FIG. 2, it is a drawing, schematically illustrating a signal transformation diagram for the hysteresis comparing device 108. In FIG. 2, the horizontal axis represents the tracking signal Va, which is received by the hysteresis comparing device 108 at the positive input terminal. The vertical axis represents digital signal Vd, which is outputed by the hysteresis comparing device 108. When the digital signal Vd is at the low voltage level and the voltage value of the tracking signal Va is gradually increasing up to an upper threshold voltage Vth, the digital signal Vd outputted by the hysteresis comparing device 108 is transformed form a low voltage level to the high voltage level. When the digital signal Vd is at the high voltage level and the voltage value of the tracking signal Va is gradually decreasing down to a lower threshold voltage Vtl, the digital signal Vd outputted by the hysteresis comparing device 108 then is transformed form a high voltage level to the low voltage level. The voltage difference between the upper threshold voltage Vth and the lower threshold voltage Vtl is called as hysteresis width. The hysteresis width usually is, for example, 20 mV. In general, when the voltage difference for a usual comparator from the positive input terminal to the negative input terminal is greater than zero, a voltage with high level is outputted. When the voltage difference for a usual comparator from the positive input terminal to the negative input terminal is less than zero, a voltage with low level is outputted. However, when the voltage difference for the hysteresis comparing device 108 from the positive input terminal to the negative input terminal is at the increasing stage, that is, the voltage difference is gradually increasing, as shown in FIG. 2 with indication of the route A. The output of the hysteresis comparing device 108 is then transformed into the high voltage level only when the voltage at the positive terminal is necessary to be greater than the upper threshold voltage Vth. When the voltage difference for the hysteresis comparing device 108 from the positive input terminal to the negative input terminal is at the decreasing stage, as shown in FIG. 2 with indication of the route D, the output of the hysteresis comparing device 108 is then transformed into the low voltage level only when the voltage at the positive terminal is necessary to be less than the lower threshold voltage Vtl. The hysteresis comparing device 108 is used to prevent glitch at the input terminal from occurring, and prevent a misjudgment on the signal.
Referring to FIG. 3, it is drawing, schematically illustrating a signal waveform for the tracking servo control loop 100. The tracking error signal generator 104 further outputs the tracking signal Va according to the satellite signals Ve and Vf. The tracking signal Va has glitch induced by the interference from the high frequency signal. Due to the hysteresis effect of the hysteresis comparing device, the glitch of the voltage value between the high threshold voltage Vth and the lower voltage threshold Vtl does not affect the outputted result from the hysteresis comparing device, so that the precision for judging the signal can be improved. When the digital signal Vd is at the low voltage level, the tracking signal Va is necessary to be greater than the upper threshold voltage Vth and when the condition is satisfied, the digital signal then is transformed from the low voltage level to a high voltage level. When the digital signal Vd is at the high voltage level, the tracking signal Va is necessary to be less than the lower threshold voltage Vtl and when the condition is satisfied, the digital signal then is transformed from the high voltage level to a low voltage level.
Referring to FIG. 4, it is a circuit diagram, schematically illustrating a circuit structure of the conventional hysteresis comparing device 108. The hysteresis comparing device 108 includes PMOS transistors PC1, PC2, PD1, PD2, NMOS transistors NL, NR, and a current source Ia with a constant direct current. All of the PMOS transistors PC1, PC2, PD1, and PD2 have the source regions that are commonly coupled to the system voltage source VDD. The gate electrode and the drain region of the transistor PD1 are electrically coupled together. The gate electrode and the drain region of the transistor PD2 are electrically coupled together. The gate electrode of the PC1 transistor is coupled to the drain region of the PC2 transistor. The gate electrode of the PC2 transistor is coupled to the drain region of the PC1 transistor. The drain region of the transistor NL is coupled to the drain region of the PC1 transistor. The drain region of the transistor NR is coupled to the drain region of the PC2 transistor, and also coupled to the constant current source Ia in cascade manner. The gate electrode of the transistor NR is the negative input terminal, and the gate electrode of the transistor NL is the positive input terminal. The hysteresis comparing device 108 has two loops. One loop is composed of the transistors PC1 and PC2 to serve as a positive feedback loop. The other one loop is composed of the transistors NL and NR to serve as a negative feedback loop. If the effect of the positive feedback loop is stronger than the effect of the negative feedback loop, then this circuit has the effect of hysteresis. This phenomenon is well known by the skill artisans. However, due to the mobility carried in the transistor, both the threshold voltage and the thickness of the gate oxide layer are easily affected or changed by the fabrication process or the operation temperature. If the transistors PC1 and PD2 are not properly matched, the transistors PC2 and PD1 are not properly matched, or the transistors NL and NR are not properly matched. Then an unstable voltage difference would be caused between the upper threshold voltage Vth and the lower threshold voltage Vtl. As a result, the width of the hysteresis comparing device 108 is not a constant.